Microsoft says it's sold more than 100 million Windows 8 licenses, but official monthly report shows precipitous decline in sales

Grab your calculator and follow along as we wind our way through Microsoft's Windows 8 sales numbers. The tale will leave you scratching your head -- or at least give you renewed respect for imaginary numbers.

While I don't put much stock in Microsoft's reported Windows sales numbers -- they're subject to a whole lot of fudging and creativity -- we have four data points where Microsoft has officially reported its Windows 8 sales statistics:

General availability of Windows 8 was Oct. 26, 2012. That's the zero point.

On May 7, in a canned (and no doubt thoroughly vetted) "interview" on the Blogging Windows blog, Reller said, "We recently surpassed the 100 million licenses sold mark for Windows 8. This number includes Windows licenses that ship on a new tablet or PC, as well as upgrades to Windows 8. This is up from the 60 million license number we provided in January."

Although there's been no explicit mention of the source behind the Nov. 27 announcement, let's assume for the sake of argument that all three published numbers were calculated the same way -- the beans didn't change between counts. Also note the precise wording in today's announcement: "recently surpassed" is not the same as "we sold 100 million as of today," so there's some wiggle room.

With those caveats, here's what has me scratching my well-scratched pate:

Between October and November 2012, Microsoft sold 37.5 million copies of Windows 8. (I've adjusted for a 30-day month.)

Between November 2012 and January 2013, Microsoft sold 14.3 million copies per month.

Between January and May, Microsoft sold 10.1 million copies per month.

Windows 7, by contrast, sold suspiciously close to 20 million copies per month every month for its entire lifetime.

Don't forget that Microsoft was able to increase its Windows 8 sales figures -- legitimately, legally -- until the end of February, by booking money that was actually received last year. There's a reason why the 'Softies chose to defer Windows revenue from 2012 to 2013.

Woody Leonhard writes computer books, primarily about Windows and Office; he's currently working on the Win 10 follow-up to the thousand-page "Windows 8.1 All-in-One for Dummies." A self-described "Windows victim," Woody specializes in telling the truth about Windows in a way that won't put you to sleep.